Woodyard D. (ed.) Pounders Marine diesel engines and Gas Turbines

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deformed by the powerful centrifugal forces acting on it; this tends to cause it

to expand across its diameter, inevitably provoking a shortening in axial direc-

tion and causing the bore to widen. The new design ensures good centring of

the wheel both on the inlet side and to the labyrinth ring in the plant system

itself. A tiebolt compensates for the axial shortening, and a tightening system

allows a small torque wrench to produce the required tiebolt tensioning. The

excellent centring makes it possible to remove and replace the compressor

wheel without the need to rebalance the rotating assembly.

Noise emission reduction was an important goal, achieved by a compact

design (the noise emitting surface area is greatly minimized) along with newly

developed insulation for the turbine outlet and compressor casing.

Noise emissions in a turbocharger occur primarily at the compressor wheel.

Any measures aimed at reducing noise therefore concentrate on the cold area.

Noise is generated by a combination of pressure ﬂuctuation in way of the com-

pressor wheel inlet and the interaction between the wheel and the diffuser. Inlet

side noise emission was reduced on the TCA series by optimizing the ﬂuid

mechanics of the compressor wheel, seeking the best compromise between

high compressor efﬁciency and noise emission. In addition, the silencer was

designed with radially arranged curve plate elements, dampening noise emis-

sions to a high degree while keeping intake pressure losses low.

In order to reduce noise radiation in the engine’s charge air manifold, the

number of blades in the diffuser was matched to that in the compressor; fur-

thermore, the compressor volute was provided with a noise-reducing casing.

All of these measures together signiﬁcantly reduced the level of noise emis-

sions compared with the NA series, and comfortably achieved the goal of

attaining emissions below the 105 dB(A) at 1-m distance level demanded by

seafarer associations and classiﬁcation societies.

A new radial silencer design is reportedly capable of reducing sound emis-

sions down to well under that level. The shape of the radially arranged damping

segments fosters optimum ﬂow conditions and contributes to total turbocharger

efﬁciency. A ﬁlter mat over the circumference of the silencer is conﬁgured for

easy removal and cleaning when required.

In the event of the worst possible form of turbocharger damage—the rup-

ture of a compressor wheel or one or more of the turbine blades—steps must

be taken to ensure that no fragments can escape from the turbocharger cas-

ings and cause injury. All the anti-burst measures used on the TCA series are

based on the idea that the forces occurring cannot be absorbed by solid, rigid

elements. The strategy instead is to employ ﬂexible elements to transform the

kinetic energy generated by the individual ruptured components into friction

heat, thereby preventing the casing components from failing as a result of local

overload.

The compressor volute, bearing casing and turbine gas outlet casing, for

example, are held together by 24 very strong tiebolts. The compressor volute is

of very rigid design, while the insert piece directly surrounding the compressor

wheel is made ﬂexible by means of long tiebolts. When the compressor wheel

Man diesel 219

220 Pressure Charging

bursts, the compressor vanes are crushed against the insert piece. The remain-

ing components of the hub body become jammed between the insert piece and

the bearing casing. The insert piece is dragged towards the silencer, using up

a major part of the kinetic energy in the component fragments. The fragments

then become lodged between the compressor volute and the bearing casing; the

remaining kinetic energy in the fragments is completely dissipated.

On the turbine side, a controlled failure point was targeted at the connection

between the turbine blade and the disc. Burst protection is therefore easier on

the turbine side than on the compressor side since, in the worst-case scenario,

one or more turbine blades can fracture when, for example, one of the engine’s

exhaust valves fractures. The components surrounding the turbine impeller are

designed to be able to absorb the kinetic energy of several broken blades.

TCr series

New standards for radial-ﬂow turbochargers were sought by MAN Diesel

in designing the TCR series, which is offered in six sizes—from TCR12 to

TCR22—to cover four-stroke engine outputs from 760 kW to 6700 kW per tur-

bocharger with pressure ratios of up to 5.2. The three largest models (TCR18,

TCR20 and TCR22) also address two-stroke engines with outputs from

2050 kW to 6400 kW.

Part- or full-load optimization is facilitated by a range of different rotor/

nozzle ring ﬂow area combinations. New CFD-optimized proﬁled rotor blades,

nozzle rings and inlet and outlet casings contribute to higher efﬁciency.

Experience from the NR series turbocharger’s radial and axial bearing

designs was tapped to beneﬁt the TCR, the ﬂoating radial bearing bushes being

coupled with new solutions such as the non-rotating radial bearings. The new

‘semi-ﬂoating’ bearing design with squeeze oil damper fosters reliable opera-

tion and low wear. The most evident modiﬁcation is the axial bearing, which is

now arranged between the radial bearings to create a space-saving design. All

bearings are housed in a bearing body.

New compressor wheel geometry underwrites improved efﬁciencies;

depending on the application, part- and full-load optimization is facilitated by

ﬂow area combinations. An internal recirculation (IRC) system can provide a

larger compressor characteristic map with increased surge margin capability.

IRC increases the stable operating range of the compressor and enables the

choice of a favourable engine operating line inside the compressor map. In this

way, it is possible to achieve higher compressor efﬁciencies at high load; this

leads to both lower fuel consumption and a reduction in mechanical and ther-

mal stress in the compressor wheel compared with a compressor having the

same pressure ratio but without IRC.

Milled from a high-resistance aluminium alloy, the compressor wheel can

withstand intake conditions within a wide temperature spectrum. A new type

of wheel attachment and retainer simpliﬁes maintenance and extends the serv-

ice life of a component subject to great stresses. An optional jet assist facility

supports rotor acceleration.

Nozzle rings are manufactured from an extremely resistant material to fos-

ter a long service life. Optimized matching of the turbocharger to a speciﬁc

engine is pursued through individually selected ﬂow areas. A variable nozzle

ring, optionally available, is recommended by MAN Diesel for engines with

frequently changing loads. Such a ring enables adaptation of the ﬂow cross-

section to the given load conditions to be optimized (see section Variable

Turbine Area technology).

As with the TCA series turbochargers, a new cast aluminium silencer low-

ers sound emissions to 

105 dB(A), and the ﬂow-optimized design of the radi-

ally arranged silencer segments contributes to turbocharger efﬁciency. The

ﬁlter mat on the silencer circumference is easily cleaned.

Again following TCA turbocharger practice, the uncooled casing of the

TCR series was designed in accordance with the ‘pipeless engine’ principle,

with all supply pipes integrated in the casing. The compressor and turbine cas-

ings feature measuring connections for monitoring the pressure and temper-

ature; a speed pick-up is also included in the scope of supply. Manufactured

from SiMo steel or ductile cast iron, the casings are designed and tested to be

containment proof, and are heat insulated to comply with Solas requirements.

Claw connections facilitate swift disassembly and allow a casing to be rotated

into any required position.

A wide inspection hatch incorporated in the turbine outlet casing offers

good access to the turbine wheel for checking and cleaning, and integrated tur-

bine and compressor washing devices are optionally available.

The DIN ﬂange of the ﬂow-optimized turbine outlet casing allows it to be

easily ﬁtted to the exhaust gas pipe; alternatively, a simple and space-saving

90° elbow can be supplied. Easier maintenance was addressed by the designers

by minimizing the number of screws and overall components, and developing

a set of special tools for servicing.

VariaBle TurBine area TeChnology

Both TCA and TCR series turbochargers are available optionally with VTA

technology, whereby a nozzle ring equipped with adjustable vanes replaces the

ﬁxed vane ring used in the standard turbochargers (Figure 7.26).

Adjusting the vane pitch regulates the pressure of the exhaust gases

impinging on the turbine, thus enabling the compressor output to be varied

and optimized at all points on the engine’s performance map. The quantity of

charge air can be more precisely matched to the quantity of fuel injected, fos-

tering reduced speciﬁc fuel consumption and emissions along with improved

dynamic behaviour of the engine–turbocharger system.

In order to minimize thermal hysteresis and improve adjustment accuracy,

each vane has a lever directly connected to a control ring, which is actuated

by an electric positional motor with integrated reduction gear. Control of the

vane position is fully electronic, with feedback or open-loop control featur-

ing mapped vane adjustment. A comprehensive range of control signals can

be used, including charge air pressure after the compressor and exhaust gas

Man diesel 221

222 Pressure Charging

temperature before and after the turbocharger. Control packages can thus be

tailored precisely to the speciﬁc application, whether that involves mechani-

cally controlled engines or engines with electronic management.

VTA adjustable vanes are manufactured from heat- and erosion-resistant

steel alloy. Careful selection of ﬁts and materials is made to ensure system

operation under all conditions without sticking, particularly in applications on

engines burning heavy fuel oil.

Shop tests and shipboard trials with VTA-equipped turbochargers have

reportedly demonstrated the expected improvements at part load in terms of

fuel economy, reduced emissions of soot and unburnt hydrocarbons, and

enhanced engine response under load changes. A lower speciﬁc fuel consump-

tion, improved torque and engine acceleration, lower combustion chamber

temperatures and savings in electrical power to drive the auxiliary blowers are

among the beneﬁts cited from the higher scavenging pressures in part-load

operation.

Retroﬁts to existing turbochargers will be facilitated by MAN Diesel sup-

plying complete packages for such projects, including the VTA nozzle ring, the

actuator and the control system.

MiTsuBishi heaVy indusTries

Successive generations of MET turbochargers introduced since 1965 by

Mitsubishi have featured non-water-cooled gas inlet and outlet casings (Figure

7.27). Demand for higher pressure ratios and greater efﬁciency was addressed

by the Japanese designer with the development of the MET-SD series, an evo-

lution of the MET-SC programme. The main features of the earlier series were

retained, but the new models exploited a more efﬁcient compressor wheel

incorporating 11 full and 11 splitter blades arranged alternately.

Figure 7.26 Man diesel’s VTa-equipped turbochargers feature a nozzle ring with

adjustable blades, illustrated here on an axial-ow TCa model

The MET-SD turbocharger shadowed the efﬁciency of the MET-SC equiv-

alent in the low-speed range but yielded 4–5 per cent more in the high-speed

range when the pressure ratio exceeds 3.5. The air, gas and lube oil pipe instal-

lations were unchanged, as were the mounting arrangements. The ﬁve models

in the MET-SD programme individually cover the turbocharging demands of

engines with outputs ranging from 1800 kW to 18 000 kW at a pressure ratio

of 3.5. The air ﬂow ratings from the smallest (MET33SD) to the largest model

(MET83SD) at that pressure ratio vary from 3.5 m

/s to 37.5 m

/s.

The MET turbocharger rotor is supported by an internal bearing arrange-

ment based on a long-life plain bearing lubricated from the main engine sys-

tem. The large exhaust gas inlet casing is of two-piece construction comprising

a detachable inner casing ﬁtted with the turbine nozzle assembly and a ﬁxed

outer casing containing the rotor shaft ﬁtted with the turbine wheel. The inner

casing can be readily unbolted and pulled from the outer casing, complete with

the nozzle assembly, thus exposing the turbine wheel and nozzle assembly

for inspection or servicing. If necessary, the turbine wheel and rotor can be

removed from the outer casing.

Mitsubishi targeted pressure ratios of 4 and beyond with the subsequent

MET-SE/SEII series, which shares the same main characteristics of earlier

MET generations:

l Completely non-cooled (no sulphuric acid corrosion of the casing)

l Inboard bearing arrangement (ease of release)

l Sliding bearings (long life).

In addition, as with previous models, an inner/outer double-walled structure

was adopted for the gas inlet casing for ease of release on the turbine side for

Mitsubishi heavy industries 223

Figure 7.27 Mitsubishi MeT-sd type turbocharger

224 Pressure Charging

maintenance checks, and a lubricant header tank is incorporated in the struc-

ture for effective lubrication of the turbocharger rotor following emergency

shutdown of the engine. MET-SE turbochargers also exploit wide chord-type

blades for improved performance in the high pressure ratio region.

The seven models in the MET-SE/SEII programme—embracing 33SE to

90SE types—deliver air ﬂow rates from 3.1 m

/s to 53 m

/s at a pressure ratio

of 3.6, satisfying engines with outputs ranging from 1300 kW to 24 100 kW per

turbocharger (Figure 7.28). Total turbine efﬁciency is enhanced by a compres-

sor impeller featuring 3D proﬁle blading to maximize aerodynamic perform-

ance, and turbine wheel blading that requires no damping wire.

Introduced in 2001 to boost two-stroke engines powering large container

ships, the MET90SE design has an air ﬂow range some 30 per cent higher than

that of the 83SE (formerly the highest capacity model) but is only 10 per cent

larger. The higher capacity from a modest increase in dimensions was achieved

by proportionally enlarging the rotor shaft and developing a new compressor

impeller wheel and turbine blades. Mitsubishi notes that expanding the capac-

ity of such wheels and blades without compromising reliability and perform-

ance is a key challenge for turbocharger designers.

Different compressor impeller materials—such as titanium and cast steel—

have been investigated as an alternative to the forged aluminium traditionally

used in MET turbochargers. A pressure ratio of 4 with an aluminium impeller

was nevertheless achieved, thanks to an impeller air cooling system and a rede-

signed compressor blade. By reducing the impeller backward curvature angle

from 35° to 25°, the pressure ratio could be raised without the need to increase

the impeller peripheral speed.

Mitsubishi successfully developed a cast stainless steel compressor impeller

in preference to an aluminium alloy component for its MET-SH turbocharger.

0 10 20 30 40 50

Air flow rate m

at 20°C

4.5

4.0

3.5

3.0

2.5

2.0

ES09

IIES38

ES38

IIES17

ES17

IIES66

ES66

IIES33

ES35

ES33

IIES33

ES24

IIES24

Compressor pressure ratio (Total-total)

Figure 7.28 air ow ranges of Mitsubishi MeT-se and seii series turbochargers

Raising the compressor pressure ratio to the higher levels increasingly

demanded by the market dictates attention to compatibility between the impel-

ler strength and design and the aerodynamic design. Mitsubishi cites, respec-

tively, the problems of higher impeller peripheral speed, elevated impeller

blade temperature and increased impeller blade surface load, and the problem

of optimizing the aerodynamic characteristics of the higher speed air ﬂow in

the impeller.

Limitations in resolving all these problems are presented by an impeller of

aluminium alloy, a material which Mitsubishi has traditionally used for MET

turbochargers because of its high speciﬁc strength and excellent machinability.

Running the aluminium alloy impeller at temperatures above the ageing treat-

ment level (190–200°C), however, threatens a reduction in material strength

and should be avoided. Depending on the suction air temperature, the impeller

blade could reach this temperature level when operating at a pressure ratio of

around 3.7.

An aluminium alloy forging also experiences progressive creep deforma-

tion at temperatures of around 160°C and above, calling for consideration in

design to preventing the stress from becoming excessive at impeller blade areas

subject to high temperature.

The stainless steel impeller casting for the SH series turbocharger deliv-

ered improved high temperature strength and made it possible to raise the max-

imum working pressure ratio to 4 and higher, without reducing the impeller

peripheral speed. Improved material strength allowed the impeller blade to be

smaller in thickness than its aluminium alloy counterpart, despite the higher

pressure ratio and increased peripheral speed. This feature in turn made it pos-

sible to increase the number of blades while ensuring the necessary throat area,

hence improving the impeller’s aerodynamic characteristics. The stainless steel

impeller of the MET42SH turbocharger, the ﬁrst production model in the MET-

SH series, features 15 solid-formed full and splitter blades.

A higher density impeller casting called for special consideration of the

rotor stability. The rotor shaft of the SH series turbocharger is larger in diame-

ter than that of the SD series, with the compressor side journal bearing located

outside the thrust bearing to increase the distance between the two jour-

nal bearings. Tests showed that, despite the higher mass of the stainless steel

impeller, the third critical speed for the rotor is around 50 per cent higher than

the maximum allowable speed, underwriting rotor security.

A turbocharger total efﬁciency of 68 per cent was logged by the MET42SH

version ﬁtted with impeller blading having a large backward angle and operat-

ing at a pressure ratio of 4.3 on a diesel genset.

Mitsubishi’s latest MET-MA turbocharger series resulted from further opti-

mization of components such as blades, gas inlet and outlet casings to yield

greater performance while retaining the proven basic construction of the previ-

ous series. An improved aerodynamic performance for increased turbocharging

efﬁciency was delivered. The turbine capacity is approximately the same as

that of the former MET-SEII series, with priority placed on higher efﬁciency.

Mitsubishi heavy industries 225

226 Pressure Charging

MET-MA turbochargers have approximately the same outer dimensions as

the MET-SE and SEII series and, apart from the gas outlet, the same interfaces

for air, gas and lube oil. Easier maintenance was sought from the detachable gas

inlet inner casing concept. Compressor design follows that of the ME-SEII series.

Eight frame sizes extend from the MET33MA to the MET90MA models

and cover the power requirements per turbocharger of engines with ratings

from 2000 kW to 27 700 kW at a pressure ratio of 3.8. The air ﬂow range of

the MA series spans 5.4 kg/s to 66.3 kg/s. An MET60MA model—suitable

for engine outputs of 10 000 kW per turbocharger—bridged the gap in size

between the MET53 and MET66 models. The MET66SE model formerly used

with engines of this rating can be replaced by the MET60MA, thus reduc-

ing the space requirement without sacriﬁcing turbocharger performance. The

MET83MA turbocharger is speciﬁed for engines requiring a pressure ratio of

around 4, and an efﬁciency of least 74 per cent has been conﬁrmed in the pres-

sure ratio range of 3–3.5; this is 2–3 percentage points higher than that for the

preceding MET83SEII model.

The gas inlet casing arrangement used for MET-SB series turbochargers

developed in 1980 and subsequently applied through to the SEII series features

a detachable inner casing. This allows maintenance to be carried out on the

turbine internals without disconnecting the engine exhaust pipe from the turbo-

charger. A smaller casing and optimized gas ﬂow were introduced in the MET-

MA series. While gas ﬂow was previously routed within the gas inlet inner

casing, the new design places the ﬂow between the inner and outer casings.

Additionally, partition plates are deployed so as to prevent gas swirling and to

facilitate a smoother ﬂow.

Velocity distribution studies indicate that ﬂow through the new gas inlet

casing is more uniform in the MET-MA turbocharger compared with the previ-

ous series, which is beneﬁcial for both greater turbine efﬁciency and reduced

turbine blade vibration. The new inlet gas casing has the extra advantage of

enabling easier inspection and cleaning of the nozzle ring since both inlet and

outlet sides of the ring are exposed when the inner casing is removed.

While the new turbine blades are of the same number and height dimen-

sions as before, the blade proﬁle was redesigned for greater turbine efﬁciency

as well as for added strength. Resonance with the number of nozzle vanes

induces substantial vibratory stress on the turbine blades. Such resonance is

particularly noticeable when the natural frequency is third mode or higher, and

the vibration mode conﬁguration is complex. Furthermore, coupled vibration

with the turbine disc must be considered. Mitsubishi notes that methods for

measuring and evaluating vibratory stress are thus important in ensuring the

reliability of newly developed turbine blades.

An asymmetric nozzle ring was developed to reduce turbine blade vibra-

tory stress in resonance with the number of nozzle vanes for application in

cases where the nozzle has low reaction (i.e., where the nozzle area is small).

The nozzle blades, which are normally spaced at uniform pitch, feature a dif-

fering pitch at approximately half the periphery. This results in the timing of

the excitation induced by the nozzle wake being interrupted at half-revolu-

tion intervals as the turbine blades rotate, thus enabling reduced turbine blade

vibratory stress during resonance. An asymmetric nozzle results in a reduction

of around 40 per cent in the maximum vibratory stress.

Asymmetric nozzle rings adopted as standard for MET-SEII and MA tur-

bochargers with small area nozzle speciﬁcations are said to offer equivalent

safety to medium and large area nozzles; they also have no impact on turbine

performance.

kBB TurBoChargers

A subsidiary of the Ogepar group, the German specialist KBB Turbochargers

(formerly Kompressorenbau Bannewitz) offers radial- and axial-ﬂow turbine

turbochargers to serve four- and two-stroke engines developing from 300 kW

to 8000 kW. The designs are characterized by compactness, inboard plain bear-

ings and both water-cooled and uncooled bearing housings.

The sixth-generation HPR series embraces four models (HPR 3000, 4000,

5000 and 6000) with radial turbines covering an engine power range from

500 kW to 3000 kW with a maximum pressure ratio of 5 and overall efﬁ-

ciencies ranging from 63 per cent to 68 per cent. Series production began in

2003, typical references now embracing Caterpillar MaK, Hyundai HiMSEN,

Wärtsilä, Yanmar and SKL engines.

KBB’s six-model radial-turbine R series addresses engines with outputs

from 300 kW to 2800 kW with maximum pressure ratios of 4 or 4.5 and overall

efﬁciencies from 62 per cent to 66 per cent.

An optional jet-assist system available for the HPR and R-series accelerates

the charger rotor on the compressor side by injecting compressed air into the

area of the impeller for a limited time span. Such a system partly overcomes

the disadvantages of single-stage turbocharging in the partial load region, and

reportedly yields a clear improvement in the start-up behaviour of the engine.

The axial-turbine M series features four models (M 40, 50, 60 and 70)

serving engines with outputs from 900 kW to 8000 kW with a maximum pres-

sure ratio of 4 and overall efﬁciencies ranging from 65 per cent to 70 per cent

(Figure 7.29).

Both axial and radial turbine series share some common features such as

internal sliding journal bearings, lubrication from the engine oil system, a

bearing life of up to 20 000 h, uncooled turbine casings and various intake cas-

ings and silencer outlines to facilitate installation on the engine. An option for

the R-3 series, variable radial turbine (VRT) geometry, delivers an extended

operating range with high torque at low engine speed, improved acceleration,

reduced smoke and lower fuel consumption.

A new generation turbocharger development by KBB was launched in

2009, the six-model ST27 radial-ﬂow series addressing high- and medium-

speed diesel and gas engines with outputs from 300 kW to 4800 kW with a

maximum pressure ratio of 5.5. Interchangeability with the previous HPR

range is reported.

kBB turbochargers 227

228 Pressure Charging

Turbocharger retrots

The beneﬁts of turbocharger retroﬁts are promoted by manufacturers, argu-

ing that new generation models—with signiﬁcantly higher overall efﬁciencies

than their predecessors—can underwrite a worthwhile return on investment.

Retroﬁtting older engines with new high-efﬁciency turbochargers can be eco-

nomically justiﬁed when one or more of the following conditions prevail,

according to ABB Turbo Systems:

l The engine suffers from high exhaust temperatures, air manifold pressure

does not rise to its normal level and full load can no longer be attained.

l Need to reduce fuel costs and/or exhaust emissions.

l Need to increase engine operating safety for an extended period.

l Engine performance requires optimizing for slow steaming or another

new operating proﬁle.

l Unsatisfactory lifetimes are experienced with key turbocharger elements

due to erosion or corrosion; older generation turbocharger spares from

some manufacturers are difﬁcult to obtain and/or are costly, and a long

delivery time is quoted for replacement turbochargers.

The performance beneﬁts from a retroﬁt (Figure 7.30) are reportedly real-

ized in lower exhaust temperatures (down by around 50–80°C) and a lower

fuel consumption (reduced by 3–5 per cent). The uncooled gas casings of new

turbochargers also foster a longer lifetime for critical parts subject to wear

from erosion and corrosion. Furthermore, ABB Turbo Systems notes that the

ball and roller anti-friction bearings normally ﬁtted to its VTR..4-type turbo-

chargers deliver better starting characteristics, improved part-load perform-

ance and faster acceleration. Enhanced reliability is also promised by modern

turbochargers.

Figure 7.29 kBB Turbochargers’ axial-turbine M-series design